This invention relates to a fluid displacement apparatus of the scroll type, such as a compressor, expander, or pump.
Scroll type fluid displacement apparatus are well known in the prior art. For example, U.S. Pat. No. 801,182 discloses a scroll type fluid displacement apparatus including two scroll members, each having a circular end plate and a spiroidal or involute spiral element. These scroll members are maintained angularly and radially offset so that both spiral elements interfit to make a plurality of line contacts between spiral curved surfaces to thereby seal off and define at least one pair of fluid pockets. The relative orbital motion of the two scroll members shifts the line contacts along the spiral curved surfaces and, therefore, the fluid pockets change in volume. The volume of the fluid pockets increases or decreases depending on the direction of the orbiting motion. Therefore, the scroll type fluid displacement apparatus is applicable to compress, expand or pump fluids. For the sake of convenience, the discussion which follows deals only with scroll type devices used as compressors.
In comparison with a conventional compressor of the piston type, the scroll type compressor has certain advantages, such as fewer parts and continuous compression of fluid. However, there have been several problems, primarily in the sealing of the fluid pockets. Sealing of the fluid pockets must be sufficiently maintained at axial and radial interfaces in the scroll type compressor, because the fluid pockets are defined by the line contacts between the interfitting spiral elements and axial contact between the axial end surfaces of the spiral elements and the inner end surfaces of the end plates.
The principles of operation of a typical scroll type compressor will be described with reference to FIGS. 1a-1d, FIG. 2 and FIG. 3. FIGS. 1a-1d schematically illustrate the relative movement of interfitting spiral elements to compress the fluid. FIG. 2 diagrammatically illustrates the compression cycle in each of the fluid pockets. FIG. 3 schematically illustrates the typical interfitting relationship of prior art spiral elements.
FIGS. 1a-1d may be considered to be end views of a compressor wherein the end plates are removed and only the spiral elements are shown. Two spiral elements 1 and 2 are angularly offset and interfit with one another. As shown in FIG. 1a, the orbiting spiral element 1 and fixed spiral element 2 make four line contacts as shown at four points A-D. A pair of fluid pockets 3a and 3b are defined between line contacts D-C and line contacts A-B, as shown by the dotted regions. The fluid pockets 3a and 3b are defined not only by the wall of spiral elements 1 and 2 but also by the end plates from which these spiral elements extend. When orbiting spiral element 1 is moved in relation to fixed spiral element 2 so that the center 0' of orbiting spiral element 1 revolves around the center 0 of fixed spiral element 2 with a radius of 0--0', while the rotation of orbiting spiral element 1 is prevented, the pair of fluid pockets 3a and 3b shift angularly and radially towards the center of the interfitted spiral elements with the volume of each fluid pocket 3a and 3b being gradually reduced, as shown in FIGS. 1a-1d. Therefore, the fluid in each pocket is compressed.
Now, the pair of fluid pockets 3a and 3b are connected to one another while passing the stage from FIG. 1c to FIG. 1d and as shown in FIG. 1a, both pockets 3a and 3b merge at the center portion 5 and are completely connected to one another to form a single pocket. The volume of the connected single pocket is further reduced by further revolution of 90.degree. as shown in FIGS. 1b, 1c and 1d. During the course of rotation, outer spaces which open in the state shown in FIG. 1b change as shown in FIGS. 1c, 1d and 1a, to form new sealed off pockets in which fluid is newly enclosed.
Accordingly, if circular end plates are disposed on, and sealed to, the axial facing ends of spiral elements 1 and 2, respectively, and if one of the end plates is provided with a discharge port 4 at the center thereof as shown in figures, fluid is taken into the fluid pockets at the radial outer portion and is discharged from the discharge portion 4 after compression.
Referring to FIG. 2, the compression cycle of fluid in one fluid pocket will be described. FIG. 2 shows the relationship of fluid pressure in the fluid pocket to crank angle, and shows that one compression cycle is completed in this case at a crank angle of 4.pi..
The compression cycle begins (FIG. 1a) with the other end of each spiral element in contact with the opposite spiral element, the suction stroke having finished. The state of fluid pressure in the fluid pocket is shown at point K in FIG. 2. The volume of the fluid pocket is reduced and compressed by the revolution of the orbiting scroll member until the crank angle reaches 2, which state is shown by the point L in FIG. 2. Immediately after passing this state, and hence, passing point L, tne pair of fluid pockets are connected to one another and simultaneously are connected to the space filled with high pressure, which is connected to the discharge chamber and is formed at the center of both spiral elements. At this time, if the compressor is not provided with a discharge valve, the fluid pressure in the connected fluid pockets suddenly rises to equal the pressure in the discharge chamber. If, however, the compressor is provided with a discharge valve, the fluid pressure in the connected fluid pockets rises slightly due to the mixing of the high pressure fluid and the fluid in the connecting fluid pockets. This state is shown at point M in FIG. 2. The fluid in the high pressure space is further compressed by revolution of the orbiting scroll member until it reaches the discharge pressure. This state is shown at point N in FIG. 2. When the fluid pressure in the high pressure space reaches the discharge pressure, the fluid is discharged to the discharge chamber through the discharge hole by the operation of the discharge valve. Therefore, fluid pressure in the high pressure space is maintained at the discharge pressure until a crank angle of 4.pi. (point O).
Accordingly, one cycle of compression is completed at a crank angle of 4.pi., but the next begins at the mid-point of compression of the first cycle as shown by points K', L' and M', and the dot-dash line in FIG. 2. Therefore, fluid compression proceeds continuously by the operation of these cycles.
Line contact between spiral elements is defined by several pairs of points as shown in FIG. 3. However, it is very difficult to attain complete contact at all points. If the line contact between spiral elements is imperfect at one or more points to form a gap, fluid leakage through the gap will occur during operation to allow the outer pockets to contain gas with higher pressure than the ideal case. The volumetric efficiency of the compressor and, hence, its refrigeration capacity will thereby be reduced. Fluid leakage across the line contact separating a pair of fluid pockets from the high pressure space is an especially very serious problem. If such leakage occurs, the pressure in the fluid pocket rises, as shown by the dotted lines and letters l, m, n in FIG. 2. Therefore, the torque or the power required in the compressing operation, is increased. As a result, the energy efficiency ratio (refrigeration capacity performed by a unit horse power) is greatly reduced. Thus, sealing of the high pressure space must be tightly secured.
The curve of the spiral elements is usually an involute curve of a circle, each spiral having the same pitch (the pitch shown as distance a.sub.1 --a.sub.2, a.sub.2 --a.sub.n, or b.sub.1 --b.sub.2, b.sub.2 --b.sub.n in FIG. 3), and these two spiral elements interfit at an angular and radial offset, so that the spiral elements make a plurality of line contacts which are represented by points a.sub.1 --a.sub.n and b.sub.1 --b.sub.n in FIG. 3. Therefore, if the pitch of the spiral element is slightly different or if the inner and outer wall curve deviates from a true involute curve due to manufacturing inaccuracies, the line contacts will be imperfect, and the apparatus which uses these spiral elements will suffer fluid leakage. In order to avoid this problem, high accuracy is required in manufacturing the spiral elements, resulting in high cost.
Even when two perfect spiral elements (having no dimensional errors) are interfitted and used in a compressor, heat developed during operation creates a thermal expansion of the elements. If the temperature is uniform throughout the spiral elements, the line contacts between both spiral elements change uniformly, and sealing of the fluid pockets is maintained. However, under actual operating conditions, thermal expansion of the spiral elements is nonuniform due to the temperature gradients, material nonuniformity or other imperfections, resulting in a nonuniform pitch variation or deviation of wall curves from a true involute. This causes a gap at the line contacts between the spiral elements, resulting in fluid leakage from the high pressure space.